This invention relates generally to optical image processing and, more particularly, to techniques for distinguishing images of distant stellar or planetary bodies from false images caused by speckles in optical imaging components, such as telescope mirrors. There is an ongoing interest in developing a reliable technique for locating Earth-like planets orbiting distant star systems. Planets have been detected largely as a result of observing perturbations in regular motions of stars, from which the presence of planets or companion stars may be inferred.
Direct terrestrial observation of planets in distant star systems is hindered by atmospheric distortion of light received in a ground-based telescope. Although use of a space-based telescope overcomes this drawback, another difficulty of direct observation of such a distant planet, even from space, is its relative proximity to the star system, in which it is located. Bright stellar radiation can saturate photodetectors associated with the telescope and make it very difficult to detect reflected light from a planet in the same field of view. One known solution to this difficulty is to employ a stellar coronagraph to occlude the stellar radiation. The solar coronagraph was invented for the purpose of observing corona activity near the sun and observing motions of planets in the solar system while masking out the sun's radiation. The same principle can be employed in a stellar coronagraph, which is designed to occlude direct radiation from a star, while imaging a surrounding field of view in which one or more planets may be directly observed.
Imaging a field of view around a target star and occluding direct radiation from the star should, in theory, produce an imaged spot wherever a planet is located in the field of view, especially if a large space-based reflecting telescope is employed. Unfortunately, however, this technique is reliably accurate only if a near perfect telescope mirror is used. Any permanent or temporary imperfection or disturbance in the mirror surface can result in the formation of a speckle in image. The speckle size will be roughly proportional to the star diameter, and the speckle magnitude (i.e., brightness) will be proportional to the magnitude of the blocked stellar radiation. The speckle location in the field of view is dependent on the size of the disturbance in the mirror surface. The relevant measure in this regard is the spatial wavelength of the disturbance. If the spatial wavelength of the disturbance is comparable to the mirror diameter, the speckle falls within the occluded stellar spot, so is not visible in the field of view. If, on the other hand, the spatial wavelength of the disturbance is, for example, one-third to one-tenth of the mirror diameter, then the speckle appears near the stellar occlusion region, where it is difficult to distinguish from a similarly sized spot due to light reflected from a planet.
One possible theoretical solution to the problem presented by mirror disturbance speckles is to build a mirror with no such disturbances. As a practical matter, this solution is virtually impossible to implement because it would require a mirror built to extremely fine dimensional tolerances. Even if the cost of such a mirror could be justified, the structure would still be subject to environmental changes, such as large extremes of temperature, that would distort the mirror surface in space. Another possible solution is to rotate the mirror about its optical axis while viewing the imaged target area. Speckles derived from mirror imperfections would rotate in the imaged field of view, but any spot derived from reflected planetary radiation would be unaffected by the rotation. Unfortunately, rotation of the mirror while maintaining accuracy in all other respects, such as pointing angle, presents significant engineering concerns, which inevitably would require equipment of additional complexity and cost.
Accordingly, there is still a significant need for a technique for imaging distant planets and, in particular, for distinguishing planetary images from speckles caused by imperfections in optical components of the telescope. The present invention satisfies this need.